Try reducing the DHW hysteresis setting and schedule the DHW to come on just before and during periods when the hot water is likely to be used. The hysteresis setting is there to stop excessive switching on the heat pump to top up the DHW (there's also the one hour default DHW anti-cycling setting which may need to be reduced). Each DHW heating cycle has a significant energy overhead in the summer months when the general heating is off and the primary circuit has to warm up from cold. This is less of a concern in the winter when the system is already warm.

One of the data files which can be downloaded using the Vaillant app contains hourly DHW temperature readings. You can scan through this to see how the system is behaving which provides useful guidance for setting the control paramenters.

Any chance that one of your bathrooms can have an electric shower installed. 

Gives you a little more flexibility with hot water reheat times.

why for what reason are we not just fitting 10kw or 12kw units, so the dhw heats as we’re using the shower at a higher speed and keep the same size cylinders?

More expensive, larger, louder and won't modulate down so low and are thus less efficient for space heating if the house doesn't need it.  If you really need six showers in close succession there are other solutions.  You are of course also at liberty to decide your own trade offs.

Yes, sorry for any confusion.  10C is sometimes quoted as the temp of incoming mains (being, I presume, roughly the temp underground at the level the mains is typically buried)

My system has the hysteresis on 5, so once it drops, it starts to kick in straight away. 

Depending on where the sensor is in the tank, leaving this feature on might well start the reheat 'automatically' sufficiently early in the 'run' of showers.  Its probably necessary to experiment to find out, although you could get a bit of an idea by estimating how far up the tank the temp sensor is.  The reheat should start when the cold water reaches this level.  Mine is about one third of the way up the (Vaillant) tank, so the reheat would start after a bit less than 2 showers.

I’m in a new build with a Vaillant 7Kw. Rads up stairs and ufh downstairs. When up stairs wants demand downstairs struggles, but back to the hot water. We came home the other night and basically had a shower one after each other but by fluke, nearly 50 mins apart. So I used then son, etc etc. but after I used it, the heating stops to heat just hot water. So my heating was off for hours.

Given the very slow response of UFH, I'm surprised that it's noticeable if the heating goes off or an hour or so. And a new build, if insulated to current standards (perhaps a big "if"), shouldn't cool down noticeably in an hour.

Depending on your system you can also schedule multiple, shorter, DHW reheats during the day/night

This can be easily done using the Vaillant app. At the moment I've got my 7kW Arotherm+ to top up the DHW in the afternoon afternoon and before 5am when the house heating gets turned up slightly. If the DHW cools down between those periods then it waits until the next scheduled top-up period.

Returnng to the original problem. I've thought of two more work-arounds which wouldn't need heating the DHW to a very high temperature:

i) Persuade some occupants to shower in the evening and just do a quick hand wash in the mornings.

ii) Run the cold supply to the shower(s) via a tank which is at room temperature or warmed by the main heating system - you'll get much more water at 40C is the cold supply is warmer.

iii) Use a bath tub and take it in turns to take a dip in the same water (as was commonplace in an earlier era).

another is to fit waste water heat recovery

I’m in a new build with a Vaillant 7Kw. Rads up stairs and ufh downstairs. When up stairs wants demand downstairs struggles, but back to the hot water. We came home the other night and basically had a shower one after each other but by fluke, nearly 50 mins apart. So I used then son, etc etc. but after I used it, the heating stops to heat just hot water. So my heating was off for hours.

Given the very slow response of UFH, I'm surprised that it's noticeable if the heating goes off or an hour or so. And a new build, if insulated to current standards (perhaps a big "if"), shouldn't cool down noticeably in an hour.

Depending on your system you can also schedule multiple, shorter, DHW reheats during the day/night

This can be easily done using the Vaillant app. At the moment I've got my 7kW Arotherm+ to top up the DHW in the afternoon afternoon and before 5am when the house heating gets turned up slightly. If the DHW cools down between those periods then it waits until the next scheduled top-up period.

Returnng to the original problem. I've thought of two more work-arounds which wouldn't need heating the DHW to a very high temperature:

i) Persuade some occupants to shower in the evening and just do a quick hand wash in the mornings.

ii) Run the cold supply to the shower(s) via a tank which is at room temperature or warmed by the main heating system - you'll get much more water at 40C is the cold supply is warmer.

iii) Use a bath tub and take it in turns to take a dip in the same water (as was commonplace in an earlier era).

OK lets go with 6, 8 minute showers at 10l/min.  So you need 480l of water at 40C.  If you start of with 300l at 48C (a common setting) and mix it with incoming mains at 10C, you will get 380l of shower water from your 300l tank.  Thats already enough for four and a bit showers.  You need to make up a further 100l for the remaining ones.

The energy required to heat this from 10 to 40C is (deltaT* specific heat capacity of water * volume) = 30*4200*100J = 12.6MJ = 3.5kWh, which a 7kW heater can supply in 30mins.  Thus being the case provided 

a) you start with a full tank

b) your tank is well stratified (the hot water is at the top) - which a modern tank should be

c) you reheat while showering

You should be able to generate enough water in the time, albeit only just.  

This being the case it may be sensible for you to think a bit about the options, rather than oversizing the heat pump.  They include

• Flick on the immersion heater in addition to the heat pump while reheating during showering, giving you 10kW total
• Heat your DHW to 58C which will give you sufficient without reheat, accepting this will cost a bit more
• Reduce shower flow rate to 8l/min
• Reduce shower time to 6 minutes
• A combination of the above
• If you have space, get a 400l tank

I think that, with a bit of thought, this problem is soluble without compromising your choice of ASHP. 

And, if I may say so without causing offence, please bear in mind that this is very much a 'first world' problem!

Do all heat pumps only do one or the other.

Any heating set up with either variable or low flow temperature does this.  In the UK we normally set up boilers to run at a high fixed flow temperature.  As a consequence they can do both at once, but the penalty is that we are (in most cases) paying ~10% more for our space heating than we need to and enjoy a lower standard of comfort!  Some other countries set boilers up to run with much like heat pumps, with variable and lower flow temperature.  This reduces cost and increases comfort, but the penalty is that only DHW or space heating can be active at any one time. 

In Germany, for example, this set up has been mandatory for many years.  Our building regulations now also specify a max space heating flow temperature of 55C which is insufficient (or at best marginal) to heat DHW so we are going in this direction.  

If you need a boost you can use the immersion - the additional cost is much less than the extra cost would be if you ran at a fixed, higher temperature.  Depending on your system you can also schedule multiple, shorter, DHW reheats during the day/night

Unfortunately there is no perfect heating set up, or if there is nobody has yet discovered it!